Electronic trip unit circuits fabricated by V.L.S.I. very large scale integration techniques, such as described within U.S. Pat. No. 4,589,052 to John Dougherty, entitled "Digital I.sup.2 T Pick-Up, Time Bands and Timing Control Static Trip Circuit Breaker", have substantially reduced the size requirements for such electronic trip circuits. The development of laminated core current sensing transformers, such as described within U.S. Pat. No. 4,591,942 to Henry Willard et al, entitled "Current Sensing Transformer Assembly", substantially reduces the cost of high quality current sensing transformers, making such current sensing transformers economically feasible within lower ampere rated industrial molded case circuit breakers. The reduction in size of both the trip unit and current sensing transformers allows the combination to be used within a common molded case industrial circuit breaker housing.
Besides providing a wide range of time over current trip characteristics, electronic trip units further provide accessory functions. Such functions include shunt trip and undervoltage as well as zone-select interlock facility, which heretofore was provided to thermal-magnetic industrial molded case circuit breakers of lower ampere rating by separate individual accessory units.
When such electronic trip units are employed with common circuit breaker operating mechanisms, a trip force must be provided to overcome the mechanical latching forces that exist between the circuit breaker cradle and the primary latch as well as between the primary and secondary latches themselves. The latching force requirements ensure resistance against so-called "nuisance tripping" due to external environmental effects, such as shock and vibration. To overcome these latching forces, a powerful trip force is usually supplied by an electromagnetic armature held against the forward bias of a relatively heavy compression spring by the attraction of a permanent magnet. An electric current pulse to the electromagnet opposes the magnetic force of attraction and allows the armature to be driven forward under the bias of the compression spring to articulate the operating mechanism and open the breaker contacts. Since the size requirement of the permanent magnet, compression spring and the electromagnet are gauged in proportion to the latching forces, such size requirements have heretofore prevented magnetic trip actuators from being economically feasible in industrial rated molded case circuit breakers of lower ampere ratings.
The present invention provides sufficient trip force to overcome the circuit breaker latching forces by the cooperative arrangement of a compact magnetic latch and a mechanical actuator. The mechanical actuator design converts a low latching force from the magnetic latch to a high tripping force at the circuit breaker trip mechanism. The low force requirements result in the use of a compact electronic trip unit, sensing transformers and accessories within a compact molded case.